In a continuous high pressure mass process for the polymerization of ethylene, compressed ethylene and a free-radical generating polymerization initiator are fed continuously to the polymerization zone while product is continuously withdrawn from the polymerization zone. As this process has been developed and refined, operators of the process have been able to modify the process to improve its efficiency or to modify the properties of the ethylene polymers being produced by employing newer types of polymerization initiators. The process also can be modified by injecting two or more different polymerization initiators into different sections of the polymerization zone. Where such techniques are employed, the polymerization initiators are selected so that they decompose and generate free radicals at different temperatures.
While such processes are employed to produce large volumes of ethylene polymers, certain undesired steps must be taken to feed the polymerization initiator to the polymerization zone. This results from the fact that most of such polymerization initiators are solids. It is virtually impossible to continously feed finely-divided solids into a high pressure reactor. As a consequence, it has been standard practice in the art to dissolve such polymerization initiators in an inert solvent and to pump such polymerization initiator solutions into the polymerization zone. As a large percentage of all high pressure ethylene polymers are employed as wrapping films for food products, the solvents employed for this purpose must have received prior F.D.A. approval. Unfortunately, many polymerization initiators of interest to the art have limited solubility in F.D.A. approved solvents. The use of dilute solutions of polymerization initiators to feed polymerization initiators to the polymerization zone concomitantly introduces significant quantities of inert hydrocarbons into the polymerization zone. This is undesirable for several reasons. For one, many of the hydrocarbon solvents employed function as telogenating agents and alter the properties of the ethylene polymers being produced. For another, the hydrocarbon solvent must be removed from the unconverted ethylene before it is recycled to the reactor. This removal step represents a significant cost burden in the process.
In addition to the general problems discussed above, certain desirable polymerization initiators have such limited solubility in F.D.A. approved solvents that they cannot be used in such ethylene polymerization processes.
In view of the above-discussed problems, it would be desirable to have available to the art improved and more versatile methods for feeding free-radical generating polymerization initiators to continous high pressure ethylene polymerization processes.